1. Field of the Invention
The present invention relates to an optical communication device for equalizing the gain-wavelength characteristics of an optical amplifier/repeater device used in a long-distance optical transmission systems. More particularly, the present invention relates to an optical equalizer for equalizing the gain-wavelength characteristics by an optical filter having asymmetrical loss-wavelength characteristics in a given wavelength range. Further, the invention relates to an optical amplifying apparatus and optical transmission system equipped with such an optical equalizer.
Ultra-long distance and large-capacity optical communication apparatuses are typically required when constructing multimedia networks. Concentrated studies are now being made of the wavelength-division multiplexing (or WDM) as a method for realizing large-capacity apparatuses in view of their advantages, namely that such multiplexing can effectively utilize a wide bandwidth and a large capacity of an optical fiber.
In ultra-long distance optical transmission systems such as transoceanic submarine cables, WDM optical signals are attenuated during transmission over ultra-long distances making it necessary to relay and amplify the WDM optical signals. A conventional repeater, having three functions of retiming, reshaping, and regeneration, have also been proposed for this purpose. However, since the optical fiber amplifier has advantages of not being dependent on the transmission speed, is capable of simplifying repeaters, and is capable of amplifying WDM optical signals as they are, the use of optical fiber amplifiers has been investigated and developed actively.
In cases where WDM optical signals are relayed and amplified by over tens of repeaters, gain-wavelength characteristics according to the amplified wavelength range are in a non-flat curve because the gains of the optical amplifiers in the repeaters differ slightly for every wavelength. Because these gain-wavelength characteristics in a non-flat curve, a deviation occurs in the optical signal/noise ratios (optical SNR) for individual wavelengths. If the deviation occurs in the optical SNR, the optical signals of low optical SNR is further deteriorated in its optical SNR while being relayed through multiple stages. For this reason, in order to reduce the gain deviation between the WDM optical signals to within a given range tolerated by the optical transmission system, it is necessary to secure a wavelength range where the gain-wavelength characteristic curve is flat.
2. Description of the Related Art
Because of the above, optical amplifiers used in repeaters and optical equalizers for flattening the gain-wavelength characteristic curve in 1550-nm wavelength range have been developed.
As shown in FIG. 13, an optical amplifier has gain-wavelength characteristics as indicated by the chain double-dashed line. If an optical filter having loss-wavelength characteristics of the same profile as the profile of the gain-wavelength characteristics (i.e., gain-wavelength characteristics opposite in profile to the gain-wavelength characteristics of the optical amplifier) can be used as an optical equalizer, then a flat gain-wavelength characteristic curve can be obtained in the 1550-nm wavelength range.
The horizontal axis of FIG. 13 indicates wavelength given in nm. The left vertical axis indicates gain in dB. The right vertical axis indicates loss in dB. The curve indicated by the chain double-dashed line indicates the gain-wavelength characteristics of the optical amplifier. The curve indicated by the dotted line indicates the loss-wavelength characteristics of an optical equalizer.
It is to be noted, however, as shown in the chain double-dotted line, that the gain-wavelength characteristics of the optical amplifier in the 1550-nm wavelength range are asymmetrical with respect to a wavelength at which the gain gives its maximal value.
The term asymmetry means that where a curve indicating the gain-wavelength characteristics is bent back about a wavelength giving a maximal value, a curve portion indicating the gain-wavelength characteristics at the shorter wavelength side and a curve portion indicating the gain-wavelength characteristics at the longer wavelength side do not overlapped.
Since the gain-wavelength characteristics have asymmetry in Japanese Patent Laid-Open No. 244079/1997, owing to the fact that an arbitrary curve can be decomposed into periodic functions because of the principle of Fourier transformation, by combining several periodic optical filters having different free spectra ranges (FSRs), an optical filter having loss-wavelength characteristics of a profile substantially identical with that of the gain-wavelength characteristics is created and the created optical filter is used as an optical equalizer.
When such several optical filters are combined, in one free spectra range (FSR) extending from one wavelength of the loss-wavelength characteristics of the optical filter that gives a certain maximum loss (minimum loss) to a wavelength giving a next maximum loss (minimum loss), that portion of the loss-wavelength characteristics which is symmetrical with respect to the wavelength giving a minimum loss (maximum loss) is utilized.
In this way, in the past, a symmetrical portion of the loss-wavelength characteristics has been utilized in order to flatten the gain-wavelength characteristic curve and, further, an optical equalizer composed of plural optical filters has been employed.
It is an aspect of the present invention to provide a method for flattening the curve of the intensity of input light over a wide range of wavelengths.
It is another aspect of the invention to provide an optical equalizer capable of flattening the curve of the intensity of input light over a wide range of wavelengths.
It is a further aspect of the invention to provide an optical amplifying apparatus capable of amplifying light over a desired wavelength range and having gain-wavelength characteristics with a flat curve over a wide range of wavelengths.
It is a yet other aspect of the invention to provide an optical transmission system capable of equalizing gain deviations coming from the loss-wavelength characteristics of an optical transmission path and from the gain-wavelength characteristics of an optical amplifier.
The above-described aspects are achieved by using, as a part to equalize gain, one optical filter utilizing the asymmetrical loss-wavelength characteristics of wavelengths. For example, in the loss-wavelength characteristics of a fiber grating filter, such an asymmetrical loss-wavelength characteristic is on the shorter wavelengths side with respect to a wavelength giving the first-order maximal value. This could also apply for the loss-wavelength characteristics on the longer wavelength side. Since the gain can be equalized by one optical filter with such a gain-equalizing part, adjustments between optical filters which are normally required in cases where gain is equalized by plural optical filters are unnecessary. Hence, simple and cost-effective optical equalizers, optical amplifying apparatus, and optical transmission systems can be provided.
It is noted that the further objects and characteristics of the present invention will be clearly shown in the following explanation described based on the accompanying drawings.